Field of the Invention
The present invention relates to a method for operating an inverter for converting direct voltage into alternating voltage, having two direct-voltage terminals and two alternating-voltage terminals, between which a plurality of power switching elements clocked at high-frequency are connected.
Inverters are used, for example, for feeding electrical energy into the public power system when only direct-voltage energy sources such as photovoltaic installations, fuel cells or batteries are available. Regardless of their type of construction, all inverters regularly absorb more energy than they deliver. It is an object of many developments of inverters, therefore, to improve their efficiency.
European Patent EP 0 284 021 B1, corresponding to U.S. Pat. No. 5,201,936, describes clock patterns for generating sinusoidal voltage and sinusoidal current for transformerless inverters. Depending on the design of their components, the efficiency of inverters of that type is best within a certain load range. Below and above that load range, the efficiency distinctly drops off.
German Published, Non-Prosecuted Patent Application DE 102 21 592 A1 discloses an inverter in which two additional switches each having one series-connected diode, are used at the output of an inverter bridge. When those switches are opened and closed at the correct time, feeding energy back from the chokes into the buffer capacitor during the commutation of the power switching elements, is avoided. The ripple of the current in the chokes, and thus their losses, become less. However, that circuit configuration requires a large expenditure of components and scarcely improves the efficiency for feeding little energy into the system.
European Patent Application EP 1 626 494 A2, corresponding to U.S. Pat. No. 7,411,802 B2, also has the object of avoiding feeding energy back into the buffer capacitor during the commutation of the power switching elements of the inverter. That is achieved by an additional switch between the buffer capacitor and the inverter bridge which interrupts the line to the buffer capacitor during the commutation. In the remaining time, however, that additional switch must conduct current and, as a result, itself again causes a loss of power. In addition, the efficiency of the inverter is scarcely improved when little energy is fed into the system.
Photovoltaic installations supply fluctuating power to the system in accordance with the fluctuating solar irradiation due to cloud coverage of the sun and diurnal variation. An inverter of a photovoltaic installation is therefore operated both in the range of good efficiency with high power delivery and over long periods in the range of poor efficiency with little power delivery. There is therefore a requirement for an inverter which can be operated effectively even at low power delivery.